There are known a number of automobile engines which include: a valve operating mechanism for selectively operating low or high speed cams and operating valves by the selected cams at proper timings so as to increase an engine output; or a valve operating mechanism for selectively operating low or high speed cams, operating valves by the selected cams, and allowing a partial cylinder operation mode by disconnecting a portion of the engine cylinders and suspending fuel supply to the disconnected cylinders, thereby increasing the engine output and reducing the fuel consumption.
A control device for such a valve operating mechanism sets various operation modes according to information concerning the operation status of the engine. For instance, in a low speed operation mode, low speed cams are used to operate the intake and exhaust valves so as to increase volume efficiency. In a high speed operation mode, high speed cams are used to operate the intake and exhaust valves so as to increase the volume efficiency.
The low or high speed cams are selectively operated in response to a cam selecting signal sent to the valve operating mechanism. Either the low or high speed cams are activated when the engine is operating at a target speed DN for selecting the low or high speed mode (called "target engine speed DN" hereinafter). The cam selecting signal is issued when the engine is operating at a given speed which is set with reference to an ordinary running condition. When the cam selecting signal is issued at the target engine speed DN while the engine is operating at a relatively low speed, either the low or high speed cams can be selectively operated without any trouble or delay. This is because the valve operating mechanism is very responsive to the cam selecting signal under such a condition.
Conversely, when the engine is idling or when the engine speed is abruptly increased due to acceleration during the operation of a low speed gears, the valve operating mechanism is slow to respond to the cam selecting signal. In such a case, the engine speed would exceed the target engine speed DN for the desired operation mode, the valve operating mechanism would fail to follow the operation mode changing, or an engine torque would become too large.